The general field of the invention relates to a common cavity electric and microwave oven and, more particularly, to such an oven having capability for self-cleaning by pyrolysis. Specifically, the invention relates to an air flow system for such an oven.
As is well known, it is desirable to force air through the cavity of a microwave oven to keep the moisture level in the cavity relatively low so that food surfaces will dry and so that moisture will not condense on the relatively cool walls of the cavity. It has been conventional to direct the magnetron blower air into the cavity through the waveguide. Not only is the waveguide a convenient entry port into the cavity but the air is frequently used to rotate a mode stirrer or primary radiating antenna positioned in or near the waveguide entrance to the cavity.
When electric heating elements are added to a microwave cavity to create what has been referred to as a "common cavity" oven, an open waveguide into the cavity can present problems. More specifically, hot air rises so that in a thermal only or self-cleaning mode when the magnetron blower is turned off, the hot air can rise into the waveguide unless its entry to the cavity is on or near the cavity floor. In fact, in prior art common cavity ovens, the microwave feed has been positioned on the floor of the cavity and, as would be expected, natural convection air flows up through the waveguide in thermal operation just as it flows in through the gap at the bottom of the door in electric only self-clean ovens. If the opening of the waveguide entrance were in the cavity at a location other than at the bottom, hot air flowing into the waveguide could overheat certain oven components such as, for example, plastic parts used to support the microwave antenna, power supply, magnetron blower, relays, etc. A typical self-clean temperature is approximately 900.degree. F. and many plastic parts start to distort at approximately 400.degree. F. Further, effluents such as grease from the food during thermal only operation and decomposition by-products during self-cleaning could tend to collect on or contaminate oven parts if an upward airflow path through the waveguide were permitted.